Sex in crowded places: population density regulates reproductive strategy.

Changing reproductive strategy from investment in current (asexual eggs) to future (sexual ephippia) reproduction depending on environmental cues is an important fitness trait in zooplankton species like water fleas, Daphnia. Different cues are reported for ephippia production, for example food limitation, changes in photoperiod and temperature, the presence of predators, nutrient depletion or signals of crowding. However, the reliability of these cues strongly depends on the local habitat. In Daphnia galeata, ephippia production might not be subject to any of the environmental cues outlined. Here, we directly test whether changes in crowding alone provides a reliable cue for D. galeata to switch reproductive mode. We conducted life history assays using 10 genotypes of D. galeata from a Swiss Lake to investigate density dependent reproduction and production of ephippia. We started populations at different densities and eliminated competition for food by systematically increasing food availability with population size. Our results suggest that population density depresses population growth rate via a change in energy allocation from current into future reproduction in D. galeata. Thus, signals of crowding produced at high population density constitute a reliable and sufficient cue for D. galeata to switch from asexual to sexual reproduction. [ABSTRACT FROM AUTHOR]